In British service, the M8 was known as the "Greyhound", a nickname seldom if ever used by the US. The British Army found it too lightly armored, particularly the hull floor, which anti-tank mines could easily penetrate (the crews' solution was lining the floor of the crew compartment with sandbags). Nevertheless, it was produced in large numbers. The M8 Greyhound's excellent on-road mobility made it a great supportive element in the advancing American and British armored columns. It was marginal off-road, especially in mud.

In July 1941, the ordnance department initiated the development of a new fast tank destroyer to replace the M6 37 mm gun motor carriage, which was essentially a ¾-ton truck with a 37 mm gun installed in the rear bed.[2] The requirement was for a 6×4 wheeled vehicle armed with a 37 mm gun, a coaxial machine gun mounted in a turret, and a machine gun in the front hull.[2] Its glacis armor was supposed to withstand fire from a .50 in (12.7 mm) machine gun and side armor from a .30 in (7.62 mm) machine gun. Prototypes were submitted by Studebaker (designated T21), Ford (T22) and Chrysler (T23), all of them quite similar in design and appearance.

In April 1942, the T22 was selected, despite complaints about deficiencies, due to the need for vehicles. By then, it was clear that the 37 mm gun would not be effective against the front armor of German tanks; so, the new armored car, now designated the M8, took on a reconnaissance role instead.[2] Contract issues and minor design improvements delayed serial production until March 1943. Production ended in June 1945.[2] A total of 8,523 units were built,[1] excluding the M20 armored utility car (see variants). The M8 was manufactured at the Ford Motor Company plant in Saint Paul, Minnesota.

In May 1942, having viewed the prototype, the British Tank Mission turned down the offer to acquire the M8 through Lend-Lease.[2] It was named "Greyhound" in keeping with other U.S. armored cars already ordered by the British, such as the (cancelled) T18 Boarhound, the T17 Deerhound, the T17E1 Staghound and the (also cancelled) M38 Wolfhound.

The cavalry reconnaissance troops (equivalent to companies and assigned to infantry divisions) and squadrons (equivalent to battalions and assigned to armored divisions or independent and used at the whim of a division or corps commander) used by the US Army served as advance "eyes and ears". This mission demanded an emphasis on speed and agility, rather than firepower and armor. When on the march, the cavalry's mission was to make contact with enemy forces at the earliest practical moment and maintain it thereafter. In this role, the recon troops identified hostile units and reported their strength, composition, disposition and movement. During withdrawals, the cavalry often served as a screening force for the main units.

The M8 performed this function with distinction. Each M8 armored car was equipped with a long-range radio set to assist in the exercise of command, or for the purpose of relaying information received from subordinate elements to higher headquarters. Another short-range radio set served to communicate within a cavalry reconnaissance platoon, reconnaissance team, or with headquarters. The M8 weighed 16,400 lb (7,400 kg) fully loaded with equipment and crew, and was capable of cruising 100–200 mi (160–320 km) cross country or 200–400 mi (320–640 km) on highways without refueling. On normal roads, it was capable of a sustained speed of 55 mph (89 km/h), hence its nickname.

The telescopic sight used to aim the main 37 mm gun

The M8 was not designed for offensive combat, and its firepower was adequate only against similar lightly armored enemy vehicles and infantry. The vehicle's armor provided a fair degree of protection against small-arms fire but nothing more. With a meager .25 in (6 mm) of floor armor, the M8 was particularly vulnerable to German mines.

The vehicle's other drawback was limited mobility in heavily wooded areas and on broken terrain; armored cavalry units preferred using the ¼-ton reconnaissance car (Jeep) in these environments. A large turning radius, limited wheel travel, open differentials, and limited cross-country mobility made the M8 armored car susceptible to immobilization off-road in off-camber terrain and defiles. This limited operators to using the vehicle mostly on existing roads and paths, where it became vulnerable to ambush. The lack of continuous tracks like a tank led to a high ground pressure and hampered its off-road performance in mud, snow and alpine terrain and the M8 frequently sank to its axles. Conversely, the performance of the M8 on hard surfaces was exceptional, with the vehicle having long range and able to consistently maintain its top speed of 55 mph. As a wheeled vehicle, the M8 was generally more reliable than tracked vehicles of similar size, and required far less maintenance and logistics support.

Shells for the main 37 mm gun are stored on racks inside the turret. The barrel of an M1 carbine, carried for close-in vehicular defense, is visible at left.

The M8's armor was thin, but it provided protection for the crew from small-arms fire and shrapnel, enough so that the vehicle could carry out its main mission of reconnaissance. The frontal, sloped hull armor varied in thickness from 0.5 inches to 0.75 inches (15.8 to 19 mm) The side and rear hull armor, also sloped but slightly less so than the front, was 0.375 inches (9.5 mm) thick. The top armor was 0.25 inches (6.4 mm) thick, as was the floor. The turret was comparatively better protected than the hull, being 0.75 inches (19 mm) thick all around, with an 0.25-inch (6.4 mm) partial roof. The cast, rounded gun shield was uniformly 1 inch (25.4 mm) thick.[3]

The M8 was fitted with a 37 mm M6 gun (aimed by an M70D telescopic sight) and a coaxially mounted .30 in (7.62 mm) Browning machine gun in a one-piece, cast mantlet, mounted in an open-topped, welded turret. The M8 was initially fitted without any kind of anti-aircraft defense; as a stopgap solution, a .50 caliber Browning M2HB machine gun on a ring mount was retrofitted to nearly all vehicles already in service. A purposefully-designed pintle was mounted on all late-production vehicles, but it saw comparatively little action due to a troubled development process.

The crew of four comprised a commander (who doubled as the loader), gunner, driver, and radio operator (who could also act as a driver). The driver and radio operator were seated in the forward section of the hull, while the commander and gunner sat in the turret, with the commander seated on the right, and the gunner on the left,

The vehicle carried 80 37 mm rounds (16 in the turret and 64 in an ammunition rack in the right sponson) when fitted with a single radio. Vehicles with a second radio installed only carried 16 main gun rounds. Some units solved this problem by cutting up the removed main ammunition rack and stowing 18 rounds in each sponson, under the radios. This raised the number of main gun rounds able to be carried to 52. Another modification (the most common one) involved fabricating (again from the discarded main ammunition rack) a 43-round bin to be placed behind the driver's seat, and a 20-round bin attached to the framing of the turret basket. This raised the ammunition capacity up to 79 rounds.[2] Machine gun ammunition consisted of 1,500 .30 caliber rounds and 400 .50 caliber rounds. In addition, the vehicle carried a mix of 16 smoke and hand grenades, four smoke pots, six M1 anti-tank mines and four M1 carbines for the crew.

The M8 was powered by a Hercules Model JXD in-line six-cylinder 320 in³ gasoline engine giving it a top speed of 55 mph (88 kph) on-road, and 30 mph (48 km/h) off-road. With a 59-gallon fuel tank and an average fuel consumption of 7.5 mpg, it could manage an average road range of 200–400 miles (320–640 km) The Hercules JXD ran more quietly than other engines of comparable power, which helped the M8 maintain an element of surprise and reduce the chance of being heard by the enemy. Because of this, the M8 armored cars in Patton's Third Army were known as "Patton's ghosts", since they were almost never detected by the Germans until it was too late.

The M8 light armored car, the "Greyhound", entered combat service with the Allies in 1943. It was purpose designed to serve as the primary basic command and communication combat vehicle of the U.S. Cavalry Reconnaissance Troops.

The M8 first saw action in Sicily in 1943 and was subsequently used by the U.S. Army in Italy, Europe, and the Pacific. In the latter theater, it was used mostly on Okinawa and the Philippines, and was even employed in its original tank destroyer role as most Japanese tanks had armor that was vulnerable to its 37 mm gun.

The vehicle was considered fast, sufficiently reliable (after some technical problems were solved) and armed and armored well enough for reconnaissance missions. However, cavalry units criticized its off-road performance, which was even worse than the M3A1 scout car it replaced[citation needed]. In the mountainous terrain of Italy and in the deep mud and snow of North European winter, the M8 was more or less restricted to roads, which greatly reduced its value as a reconnaissance vehicle. It was also very vulnerable to landmines. An add-on armor kit was designed to provide an extra quarter-inch of belly armor to reduce landmine vulnerability. Some crews placed sandbags on the floor to make up for the thin belly armor.

Another problem was that commanders often used their reconnaissance squadrons for fire support missions, for which the thinly-armored M8 was ill-suited. When it encountered German armored reconnaissance units, the M8 could easily penetrate their armor with its 37 mm gun. Conversely, its own thin armor was vulnerable to the 20 mm autocannons that German scout cars were equipped with.

During the Battle of St. Vith in the Battle of the Bulge, an M8 of Troop B, 87th Cavalry Reconnaissance Squadron was able to destroy a German Tiger II heavy tank. The M8 fired three 37 mm rounds through the relatively thin rear armor of the Tiger from only 25 yd (23 m), setting it on fire.[4]

The U.S. Army started to look for a replacement for the M8 as early as 1943. Two prototypes, the StudebakerT27 and ChevroletT28 were finished in summer 1944. Both were found to be superior to the M8, but it was decided that, at that stage of the war, there was no more need for a new armored car.

After the war, the M8 was used for occupation duty; it also saw combat in the Korean War, being retired by the U.S. Army shortly thereafter. In French use, the M8 was used during the Indochina War (1946–1954) and Algerian War (1954–1962). Many vehicles formerly used by the U.S., Britain and France were exported to NATO allies and third world countries. As of 2002, some still remained in service in Africa and South America.

The M8 was in service after World War II in Austria, first with the B-Gendarmerie constabulary force, then later with the Bundesheer, established in 1955. Austrian M8s in B-Gendarmerie service had the main gun removed. When these vehicles were absorbed into the newly-created army, they were rearmed with M2 Browning .50 Caliber machine guns and 37mm cannons purchased as surplus.[5]

The M8 was used in the Congo, first as the Belgian colonial Force Publique's only armored asset, then with the Congolese Army following independence in June, 1960.[6] During the Congo Crisis, Indian peacekeepers with recoilless rifles destroyed at least one ex-Belgian Greyhound manned by Katangese separatists. The armored cars were deployed on both sides during UN attempts to end Katanga's ill-fated secession.[7]

Several Greyhounds were deployed in Bogota on March 8, 2007, as part of the security measures for U.S. President George W. Bush's visit. They are regularly used as checkpoint security by the Colombian military, and usually can be seen in the northern parts of the capital.

M8E1 light armored car - a variant with modified suspension. Two vehicles were produced in 1943.

The M20 armored utility car, also known as the M20 scout car, was a Greyhound with the turret replaced with a low, armored open-topped superstructure and an anti-aircraft ring mount for a .50-in M2 heavy machine gun. A bazooka was provided for the crew to compensate for its lack of anti-armor weaponry. The M20 was primarily used as a command vehicle and for forward reconnaissance, but many vehicles also served as armored personnel carriers and cargo carriers. It offered high speed and excellent mobility, along with a degree of protection against small arms fire and shrapnel. When employed in the command and control role, the M20 was fitted with additional radio equipment. Originally designated the M10 armored utility car, it was redesignated M20 to avoid confusion with the M10tank destroyer. 3,680 M20s were built by Ford during its two years in production (1943–1944).

T69 multiple gun motor carriage - in late 1943, an anti-aircraft variant of the M8 was tested. The vehicle was armed with four .50-in machine guns in a turret developed by Maxson Corp.. The antiaircraft board felt that the vehicle was inferior to the M16 MGMC and the project was closed.

M8 TOW tank destroyer - M8 upgraded by the US company Napco. The main gun was replaced by an .50-in machine gun and a BGM-71 TOW launcher was installed above the turret. Upgraded vehicles were used by Colombia.

M8/M20 H-90 - a French upgrade for the M20 showcased by GIAT Industries in 1971, which mounted a 90mm low-velocity gun adopted from the Panhard AML family of reconnaissance vehicles.

CRR Brasileiro - a version developed in 1968 by the Brazilian Army Engineering Institute (IME). The middle axle was removed and a new engine (120 hp (89 kW) Mercedes-Benz OM-321) installed to create the VBB-1 of which one prototype was completed, the vehicle being found to be inferior. The Vbb-1 was, in turn, the basis for the CRR which reverted to a 6×6 configuration and eight vehicles were produced for evaluation. The EE-9 Cascavel was developed from the CRR.

M8 (diesel) Hellenic Army armored car - a number of M8 armored cars were upgraded with a Steyr diesel engine in place of the Hercules JXD gasoline engine, this required a rearwards extension of the engine compartment by 11.8 in (300 mm), as well as some heightening. Also fitted were a new radio, indicator and new hooded lights, rear view mirrors, while the M2HB anti-aircraft machinegun was moved to the right front of the turret, where a new pintle socket was bolted on the partial roof (the turret rear socket being retained) and the coaxial 0.30-in M1919A4 replaced by a 7.62×51mm NATO MG3 machinegun. Used for coastal defense and retired from service in the late 1990s.

Colombian AM8- a Colombian fusion of turret-mounted World War II anti-air artillery in an M8 with a modern motor. It is a counter-insurgency weapon for use against guerrilla ambushes in the Colombian mountains.